It has been known that rolling resistance can be reduced by lowering the tan δ of a rubber composition used for the sidewall portion and clinch portion of a tire.
At that time, sulfur is used as a vulcanizing agent and a vulcanization accelerator is further used (for example, refer to Japanese Unexamined Patent Publication No. 2006-63143). However, it is known that sulfur is discharged at a final kneading step at low temperature and dispersibility is bad. Further, when the dispersion of sulfur is poor, a rubber is ununiformly vulcanized and tan δ and fracture strength is deteriorated.
Further, those such as HTS and PK900 available from Flexsys Chemicals Sdn. Bhd. and KA9188 available from Bayer AG that are hybrid crosslinking agents are used as an aid in order to suppress reversion and it is also known that a certain level of effect is obtained. However, although sulfur is partially included in HTS, fundamental solution is not carried out yet.
In order to solve the problem, methods such as oil treatment, the use of insoluble sulfur, the use of master batch of polymer and sulfur, and the use of sulfur pellet chemical (obtained by blending sulfur and a binder rubber, for example, at a ratio of 50:50) are carried out.
Further, it is also known that an alkylphenol-sulfur chloride condensate is used as the crosslinking agent of a white rubber and a butyl rubber of a tire. The alkylphenol-sulfur chloride condensate is easily scorched. Because the white rubber and butyl rubber have originally long scorch time, problems such as rubber scorch and surface bloom are not generated even if the alkylphenol-sulfur chloride condensate is used. On the other hand, since the scorch time of rubbers such as a natural rubber usually used for sidewall and clinch is not so long as the white rubber and a butyl rubber, there arises a problem such as rubber scorch during processing when the alkylphenol-sulfur chloride condensate is used as a crosslinking agent. Further, when a large amount of PVI (a retarder CTP available from OUCHISHINKO CHEMICAL INDUSTRIAL CO., LTD.) being a vulcanization retarder is compounded, there arises a problem such as the surface bloom of PVI itself. Consequently, when the alkylphenol-sulfur chloride condensate is used for sidewall and clinch, on which surface bloom is not acceptable, the PVI is required to be used for suppressing scorch. And it has been considered that PVI cannot be used because of the surface bloom.